Tumour therapy using particle beams, especially using protons, a particles and carbon nuclei, is increasing in importance. In that context, the raster scanning method has a number of advantages over other methods. In this method, a three-dimensional raster is placed over the tumour volume. The particle beam is deflected by deflecting magnets in two directions perpendicular to one another (the x and the y directions). By actively varying the particle energy, the position of the Bragg peak, in which the largest portion of the dose is deposited, is set at differing depths (z direction) in the body.
The dose which is to be administered as a function of the location is specified or prescribed by a user after thorough diagnosis. For example, the dose within the tumour should be as constant as possible and should fall off as steeply as possible outside the tumour. A more complex dependency of the dose on the location is also possible, however. As accurate as possible adherence to the dose is an important prerequisite for successful treatment.
Discrepancies between the dose actually administered and the dose prescribed, which is referred to hereinbelow also as the desired dose, can come about as a result of a variety of causes. Such causes include inter alia discrepancy between the location of the particle beam and the intended location and also movements of the patient or parts of the patient during the irradiation.
In order to reduce the effects of such influences, there are chosen inter alia as fine as possible a raster of target points of the particle beam and as large as possible an extent of the particle beam. The finer the raster, however, the longer the duration of irradiation, because every change in the deflection of the particle beam in the x or y direction and every change in the particle energy requires a certain amount of time. In particular, a change in the particle energy requires numerous changes and adjustments to the settings of magnets of the accelerator and the beam transport unit. Those changes and adjustments require a period of time which has a significant effect on the total duration of the irradiation. The larger the spatial region over which the dose administered or produced by direction of the particle beam at a selected target point, the more shallowly the dose falls off at the edge of the target volume. The larger the cross-section of a particle beam, the lesser is also the accuracy with which its position can be monitored, for example by means of a multi-wire proportional counter. Accordingly, a compromise is necessary in respect of spacings between isoenergy layers and the breadth of the dose distribution resulting from the direction of the particle beam at a single target point.
Similar problems exist not only in the case of irradiation of a tumour in a patient, but also in the case of many other applications in which a target volume in any (animate or inanimate) body is irradiated with a particle beam. Examples that may be mentioned are the irradiation of anatomical models in the context of research work or in the context of the quality checking or quality assurance of a system and the irradiation of materials in materials research or for modifying the properties thereof.